I had a PC whose Sound Blaster 16 PCI was overloading on line-level input
signals, even with the recording level set at the very bottom of the scale.

The input was measured at 3 volts peak-to-peak (1 volt rms), which is a
little high but not excessive for line level.

It turned out to be a driver problem. The Windows driver for this card does
not give the full range of level adjustment. The Creative Labs driver, even
though older, solved the problem.